A method of purifying the electrolyte salt employed in an electrochemical cell

ABSTRACT

A process is described for increasing the energy storage capacity and cell like of an electrical energy storage device comprising a porous carbon cathode, a lithium alloy anode and a fused salt electrolyte composed of an alkali metal halide or an alkaline earth metal halide or mixtures thereof, wherein the electrolyte salt is purified electrochemically before use and is subsequently circulated to the electrochemical cell.

United States Patent Inventor Harry A. Adams Bedford Heights, Ohio Appl.No. 848,219 Filed Aug. 7, 1969 Patented Sept. 21, 1971 Assignee TheStandard Oil Company Cleveland, Ohio A METHOD OF PURIFYING THEELECTROLYTE SALT EMPLOYED IN AN ELECTROCHEMICAL CELL 5 Claims, 2 DrawingFigs.

[1.5. CI. 136/153, 136/ l 55 Int. Cl H0lm1l/00 Field ofSearch 136/153,

Primary Examiner-Winston A. Douglas Assistant Examiner-A. SkaparsAuomeys-lohn F. Jones and Sherman J. Kemmer ABSTRACT: A process isdescribed for increasing the energy storage capacity and cell like of anelectrical energy storage device comprising a porous carbon cathode, alithium alloy anode and a fused salt electrolyte composed of an alkalimetal halide or an alkaline earth metal halide or mixtures thereof,wherein the electrolyte salt is purified electrochemically before useand is subsequently circulated to the electrochemical cell.

I H SEPZI IHYI GF'I'IIFIII I IR L sALT FIG. I

' I I0 LOADING k CHAMBER 2O DRY BOX GRANULAR sALT I FORMATION Q} TANK IREAOTOR Q2) I f\ I2 V III a? FILTER DUMP TANK I IMPURE GRANULAR FIG 2IO\ D 20 LOA ING CHAMBER DRY BOX GRANULAR SALT FORMATION l6 f TANK I2FILTER I d5 DUMP TANK I INVENTOR. I8 HARRY A. ADAMS fah 4 jam ATTORNEY AMETHOD OF PURIFYING THE ELECTROLYTE SALT EMPLOYED IN AN ELECTROCHEMICALCELL SALT PURIFICATION AND CIRCULATION SYSTEM This invention relates toa method for increasing the energy storage capacity and the cell life ofan electrical energy storage device. More particularly, this inventionrelates to a method for increasing the energy storage capacity and thecell life of an electrical energy storage cell comprising a porouscarbon cathode, a lithium alloy anode, and said electrodes beingimmersed in or in contact with a fused salt electrolyte composed of analkali metal halide or an alkaline earth metal halide, or mixturesthereof.

Essentially, this invention comprises a method of treating theelectrolyte salt employed in an electrochemical cell to removeimpurities therefrom, and a means for continuously circulating thetreated molten salt to the electrochemical cell. A further aspect ofthis invention comprises the use of the treated salt as an electrolytein the electrochemical precondi' tioning" of the carbon electrode toimprove its electrical energy storage capacity.

Heretofore, commercially available salts have been virtuallyunacceptable for use as electrolytes in fused salt energy storagedevices because of the presence of varying amounts of impurities. Theexact nature of these impurities is not known, however, small amounts ofmoisture, adsorbed gases and trace to small amounts of various othermetals and metal oxides are known to be present. These impurities havean adverse effect on cell life and efficiency of the electrochemicalcell, by causing increased leakage current and corrosion of the cell,and a lowering of the decomposition potential of the electrolyte. Theseadverse effects can minimized by the utilization as an electrolyte aslat that has been treated in a manner as described herein.

Preconditioning of the electrodes as referred to in this inventionconstitutes the removal of easily degradable components in the structureof the electrode and the permeation of the electrode with electrolyte byalternately charging in a positive direction and then discharging in anegative direction while in contact with the electrolyte. These stepsamount to electrochemical oxidation and reduction reactions and resultin changing the carbon structure by removing substantially all of theoxygen and hydrogen and most of the ash from the electrode.

in accordance with the process of this invention, the harmful impuritiesnormally present in commercial grades of salts such as alkali metalhalides and alkaline earth metal halides are removed by electrochemicaltreatment of the fused salt. Briefly, in this particular embodiment, theimpure, granular salt is placed in a loading chamber where excessmoisture is removed by heating the salt to a temperature below itsmelting point under an inert atmosphere and reduced pressure. Afterdrying to a constant low level moisture content, the granular salt isconveyed to the reactor where it is then subjected to electrolysis underan inert atmosphere and reduced pressure to remove the impurities.Following this treatment the purified molten salt flows from the reactorto the filter where any ag' glomerates or oversized particles areremoved. The salt is then conveyed to the formation tank where thecarbon cathode to be used subsequently in a molten salt energy storagedevice is preconditioned by alternately charging in a positive directionand discharging in a negative direction while immersed in the purifiedelectrolyte salt. The resulting impure molten salt containing impuritiesreleased from the carbon cathode in the formation tank then flows bygravity into the dump tank. Upon pressurizing the dump tank, the saltflows back into the reactor through the transfer line indicated forfurther purification and is filtered and returned to the formation tank.A portion of the purified salt is transmitted to the dry box for use asan electrolyte in an electrochemical cell, while a separate portion canagain be utilized in the formation tank in the preconditioning ofanother carbon electrode. The process described in this invention may beoperated continuously or intermittently.

The degree of contamination of the salt to be purified is not a criticalfactor in this invention since, irrespective of the amount of theimpurities initially present, treatment in accordance with the processdescribed results in improved performance of the salt. Generally,however, salts containing from 0.3 to 10 percent by weight of impuritiesmay be treated advantageously, and the degree of purification obtainedis proportional to the duration of the treating time. The salt isconsidered to be sufficiently pure when a maximum voltage is reached fora particular electrochemical system on electrolyzing the salt atconstant current. For example, this qualification is met for a eutecticmixture of lithium chloride-potassium chloride by electrolyzing the saltat a constant current of 63.5 milliamperes per inch of cathode until apotential of 3.335 volts is achieved in a cell containing a graphitecathode, an aluminum-lithium alloy anode and which is operated at atemperature of 500 C. a pressure of l p.s.i. absolute, in an argonatmosphere.

Although as previously stated the exact nature of the impurities removedis not definitely known, gaseous materials such as water, chlorine,hydrogen, oxygen, nitrogen, methane, carbon dioxide and carbon monoxidehave been detected at the electrodes of the purification cell and metaloxides are reduced on the surface of the anode.

Although this process is applicable to the purification of metal saltsof alkali metals, alkaline earth metals, or mixtures thereof, whereinthe anion is selected from the group consisting of nitrates, nitrites,or halides, or metal salts of alkali metals wherein the anion is acarbonate group, the process is particularly applicable to salt mixturesof halide salts of alkali metals and alkaline earth metals because theyare most suitable for use in the energy storage cell described herein.Fused salt mixtures containing, for example, sodium chloride, calciumchloride, calcium fluoride, magnesium chloride, lithium chloride,potassium chloride, lithium bromide and potassium bromide can be used.The lower melting point electrolytes are desirable. Specific examples ofuseful [binary salt electrolytes are lithium chloride-potassiumchloride, lithium bromidepotassium bromide, lithium fluoride-rubidiumfluoride, lithium chloride-lithium fluoride, lithium chloride-strontiumchloride, calcium chloridedithium chloride, lithium sulfatepotassiumchloride, and mixtures thereof.

Examples of ternary molten salt electrolytes useful in this inventionare calcium chloride-lithium chloride-potassium chloride, lithiumchloride-potassium chloride-sodium chloride, calcium chloride-lithiumchloride-sodium chloride, and lithium bromide-sodium bromide-lithiumchloride.

Especially preferred systems are those of potassium chloride-lithiumchloride and lithium bromide-potassium bromide and mixtures thereof. Alithium chloride-potassium chloride system of 41 mole percent potassiumchloride and 59 mole percent lithium chloride forms a eutectic whichmelts at 352 C.

This invention can be employed where highly purified alkali metal andalkaline earth metal halide salts are essential for certain other usesin addition to their specific use in electrical energy storage devices.

A preferred embodiment of this invention is illustrated by the schematicflow diagram shown in FIG. 1 wherein the impure granulated salt is addedto the loader 10 and is heated to a temperature in the range from aboutC. to about 50 C. below its melting point and generally not above 300 C.The loader is designed to operate within the range of full vacuum to 30psi. absolute. As heat is applied to the salt, an inert gas such as, forexample, argon or helium is swept across the surface of the salt tocarry off the surface adsorbed gases and moisture that are driven off onheating. The length of time required for drying is determined bychecking the amount of moisture present in the off gas at various timeintervals. When the concentration of water vapor in the off gas fallsbelow 35 p.p.m. at a gas sweep rate of 5 s.c.f./hr. the salt isconsidered to be essentially dry. The drying period may require from oneto several days. The pressure in the loader is then brought toatmospheric pressure with the inert gas and the salt is transmitted tothe reactor 12.

The reactor vessel 12 is designed to operate at pressures ranging fromfull vacuum to 65 p.s.i. absolute and temperatures of up to 650 C. Thesalt is maintained in the molten state in the reactor and means isprovided for continuous agitation of the salt while the purification istaking place. An inert atmosphere, preferably argon, is also provided.The electrolysis unit consists of a porous carbon or graphite cathode, alithium alloy anode, and the salt being purified functions as theion-conducting medium.

In general, the carbons utilized in the preparation of the cathode maycomprise such materials as activated petroleum coke, wood char,activated sodium lignosulfonate char, activated bituminous coal,polyvinylidine chloride char, polyacrylonitrile char and the like. Thegraphite composition may comprise any commercially available graphiteprepared from any one of the foregoing types of carbon.

The anode of this reactor comprises lithium or a lithium alloy such asaluminum-lithium, indium-lithium, tin-lithium, lead-lithium,silver-lithium, copper-lithium, etc. Ternary lithium alloys can likewisebe used.

The preferred anode is the highly reversible aluminum-lithium electrode.This electrode can be produced by combining lithium with aluminum andthus producing a preformed alloy of aluminum and lithium, or,electrochemically, by charging a substantially pure aluminum bar in anelectrolyte containing lithium ions to the extent of about one amperehour per gram of aluminum, whereby lithium is diffused into the aluminumbar.

The preferred aluminum-lithium alloy anode comprises aluminum andincidental impurities in amounts of from about 70 to 95 weight percentbased on total composition, and from to 30 weight percent lithium.

In the electrolysis step of the instant process, the cell is charged ata constant current of, for example, 63.5 milliamperes per inch" ofcathode to a maximum voltage up to the decomposition potential of theparticular salt being electrolyzed. During the reaction the gaseousimpurities in the salt are evolved at the electrodes and the metallicimpurities are deposited on the surface of the anode. Upon completion ofthe treatment of the salt, the molten salt is conveyed from the reactorto the filter 14 where most of the solid material is removed. Pressureof approximately 35 p.s.i. absolute of argon is applied to force thesalt through the filter and on to the formation tank 16. The circulatingmolten salt is then transmitted to the formation tank 16 where thecarbon cathode to be used subsequently in a molten salt electricalenergy storage cell is preconditioned before use. The counteraluminum-lithium electrode in the formation tank is reused forsubsequent preconditioning of carbon cathodes. An inert atmosphere ismaintained in the formation tank and the tank is capable of beingoperated at full vacuum to 14.8 p.s.i. absolute of pressure and attemperatures of up to 650 C. After the preconditioning treatment hasprovided the desired state of charge, the electrode is removed from theformation tank and placed in a battery container and the remainingelements assembled.

On completion of the preconditioning of the cathode, the electrolytesalt now containing impurities removed from the electrode is circulatedto the dump tank 18. The dump tank is sufficiently large to hold thesalt from the reactor 12 and the formation tank 16. This tank serves asa means for recirculating the salt from the formation tank to thereactor for reprocessing and back to the formation tank. It also servesas an emergency dump container to receive any overflow from theformation tank. The tank is capable of being operated under the sametemperatures and pressures as employed in the reactor. The vapor phaseof the dump tank is connected with the vapor phase of the formation tankso that equal pressures are maintained to insure proper gravity flow.The salt from the dump tank is then pressurized to cause flow to thereactor where it is again purified. The purified salt from the reactoris recirculated tothe filter 14, then to the formation tank where itagain can be utilized in the preconditioning of other carbon electrodes,or, if desired, it may be conveyed to the dry box 20 where it isresolidified and maintained under a purified inert atmosphere at atemperature below about 300 C. for use in an electrochemical cell.

In another embodiment of this invention, the salt treatment andpreconditioning of the carbon cathode may be accomplished simultaneouslyin a single electrolysis operation carried out in the formation tank. Insuch an arrangement, shown in the schematic diagram in FIG. 2, thereactor vessel is bypassed and the solid salt is conveyed from theloader 10 to the dump tank 18 where the salt is heated to at least 50C.above its melting point and the molten salt is then passed through thefilter l4 and then to the formation tank l6.where, in theelectrochemical treatment, gaseous impurities from the electrolyte andthe carbon cathode are evolved at the electrodes and metal impuritiesare deposited on the anode.

It is essential that the materials employed in the construction of thecomponent parts of the system described are resistant to the thermal andchemical degradation caused by the molten salt. For purposes of thisprocess, stainless steels of the 300 series are suitable, and mostpreferred are the 316 and 347 stainless steels. The treated salt is lesscorrosive than the untreated salt so that component parts of theprocessing equipment coming in contact with the purified salt can beconstructed from lower cost materials such as, for example, the carbonsteels.

A better understanding of the invention may be obtained from thefollowing specific example.

EXAMPLE The assembly unit employed in this example included a maincontrol panel to control the temperatures and pressures throughout theunit. An atmosphere of argon was maintained throughout the system. Theflow rate of the salt was controlled by the pressure differences fromone vessel to another. All components of the system coming in contactwith the electrolyte salt were constructed from 347 stainless steel.

In this particular example, 25 pounds of a granular electrolyte saltcomprising a eutectic mixture of 41 mole percent lithium chloride and 59mole percent potassium chloride were placed in a loader 10 having a7-gallon capacity. Before treatment the electrolyte salt contained thefollowing impurities as determined by spectrographic analysis:

water 0.49?

Iron 0.002% Chlorine (as C10 0.003T Nickel major Aluminum l0 p.p.m.Copper 10 p.p.m. Magnesium l0 p.p.m. Manganese l0 p.p.m. Silicon l0p.p.m. Chromium trace Niobium trace Phosphorus trace Titanium trace Theloader was operated at a temperature of l77 C. and a pressure ofapproximately 2.5 p.s.i. absolute. The loader was purged with argon atthe rate of 5 s.c.f./hr. to carry off excess moisture and other surfaceadsorbed gases given off during the heating of the salt. The off gas wasanalyzed continuously until the water content remained constant at about35 p.p.m. The loader was then repressurized with argon to atmosphericpressure to transmit the salt to the reactor. The reactor l2-had acapacity of 20 gallons and was equipped with a stirrer to continuouslyagitate the salt during the purification step. An electrolysis unitconsisting of a graphite cathode and an aluminum-lithium alloy anode (18weight percent lithium) was added through the top entry port of thereactor. The reactor was purged with argon and was operated at atemperature of 538C. and a pressure of approximately 1 p.s.i. absolute.The molten electrolyte salt was electrolyzed at a constant current of63.5 milliamperes per inch of cathode for about 48 hours until apotential of 3.34 volts was achieved. Spectrographic analysis of thecomposition of the electrolyzed salt indicated the absence of chromium,copper, iron, manganese, nickel, niobium, phosphorus and titanium andthe presence of trace amounts of aluminum, magnesium and silicon.Following the electrolysis, the reactor was pressurized with argon totransmit the purified salt from the reactor to the filter l4 and apressure of approximately 35 p.s.i. absolute was applied to convey thesalt through the filter. The salt was then transmitted to the formationtank 16 and subsequently to the dry box 20.

The purified salt was subsequently utilized as an electrolyte in amolten salt electrical energy storage cell employing a porous carboncathode and an aluminum-lithium alloy anode. The cell exhibited anelectrical energy storage capacity of 2 ampere-hours per cubic inch ofcarbon cathode as compared with a capacity of 1.0 to 1.5 ampere-hoursper cubic inch of carbon cathode obtained for a cell utilizing theuntreated salt.

Iclaim:

l. A method for increasing the electrical energy storage capacity of amolten electrolyte salt electrical energy storage device wherein theelectrolyte salt is subjected to the following sequence of treatingsteps prior to use in said electrical energy storage device:

1. heating to a temperature of from about 100C. up to about 50C. belowthe melting point of the salt in a substantially oxygen-free environmentand at a pressure of about 0.1 to 30 psi. absolute and removing thevolatile components from the salt by continuously purging the vesselwith an inert gas;

2. melting the salt resulting from step (1) at a pressure of from about0.1 to 65 psi. absolute, in a substantially oxygen-free environment,immersing into the molten salt a pair of spaced opposed electrodescomprising a carbon cathode and a lithium alloy anode, and electrolyzingsaid molten salt at a constant current until a predetermined maximumvoltage is achieved; and

3. separating any solid material from the molten salt.

2. The method of claim 1 wherein the :anode in step (2) consistsessentially of an aluminum-lithium alloy.

3. The method of claim 2 wherein the anode consists essentially of analloy of 70 to weight percent of aluminum and 5 to 30 weight percent oflithium.

4. The method of claim 3 wherein the electrolyte salt is a mixture oflithium chloride and potassium chloride.

5. The method of claim 4 wherein the electrolyte salt is a eutecticmixture of lithium chloride and potassium chloride.

" UNITED STATES PATENT OFFICE 3, 7, September 21, 1971 Patent No. DatedInventor(s) Harry A Adams It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Abstract: Line 2; "like" should be --life-- Column 1: Line 3, old title"Salt Purification and Circulation System" should be eliminated Column3: Line 38, "inch should be ---inch Column A: Line 52, "010 should be---c1o second column "0.003'1" should be Signed and sealed this 28th dayof March 1 972.

(SEAL) Attest EDWARD M.FLETCHER, JR. ROBERT GOTTSCHALK Attesting OfficerCommissioner of Patents

1. A method for increasing the electrical energy storage capacity of amolten electrolyte salt electrical energy storage device wherein theelectrolyte salt is subjected to the following sequence of treatingsteps prior to use in said electrical energy storage device:
 2. meltingthe salt resulting from step (1) at a pressure of from about 0.1 to 65p.s.i. absolute, in a substantially oxygen-free environment, immersinginto the molten salt a pair of spaced opposed electrodes comprising acarbon cathode and a lithium alloy anode, and electrolyzing said moltensalt at a constant current until a predetermined maximum voltage isachieved; and
 2. The method of claim 1 wherein the anode in step (2)consists essentially of an aluminum-lithium alloy.
 3. The method ofclaim 2 wherein the anode consists essentially of an alloy of 70 to 95weight percent of aluminum and 5 to 30 weight percent of lithium. 3.separating any solid material from the molten salt.
 4. The method ofclaim 3 wherein the electrolyte salt is a mixture of lithium chlorideand potassium chloride.
 5. The method of claim 4 wherein the electrolytesalt is a eutectic mixture of lithium chloride and potassium chloride.